1. Field of the Invention
The present invention relates to communications based on phase-based modulation, such as bipolar-phase shift-keying (BPSK) modulation or quadrature-phase shift-keying (QPSK) modulation.
2. Description of the Related Art
In a typical fiber-to-the-curb (FTTC) telecommunication system, signals are transmitted between a central office (CO) and one or more remote nodes as optical signals over optical fibers, while signals are transmitted between each remote node and one or more consumer product equipment (CPE) units (e.g., telephones, faxes, computers) as electrical signals over copper wires. As such, each remote node has components for receiving downstream optical signals from the central office, converting the optical signals into electrical signals, and transmitting the electrical signals to the CPE units. Similarly, each remote node has components for receiving upstream electrical signals from the CPE units, converting the electrical signals into optical signals, and transmitting the optical signals to the central office.
In certain implementations of an FTTC telecommunication system, at least some of the signals received by the remote nodes are signals that have been modulated using bipolar-phase shift-keying (BPSK) or quadrature-phase shift-keying (QPSK) modulation. For example, the upstream electrical signals received by the remote nodes may be BPSK/QPSK signals. In such an implementation, each remote node has one or more BPSK/QPSK receivers for receiving the BPSK/QPSK signals for subsequent conversion from electrical to optical and further transmission.
In BPSK/QPSK modulation, information is encoded in the phase of the signals. In order to convert BPSK/QPSK signals, a receiver samples the signal stream at a sufficiently high rate to detect the locations of the zero crossings in the signal from which the phase of the signal can be derived. In order to keep costs down and in order to provide a low-power remote node for FTTC applications that withstands a wide range of operating conditions, it is desirable to implement a BPSK/QPSK receiver using hard-limiting techniques that resolve only the most significant bit (i.e., the sign bit) in the sampled signal level, thereby determining whether the sample is positive or negative. The phase of the BPSK/QPSK signal can then be derived from the locations of transitions between positive and negative samples (i.e., the zero crossings).
In general, BPSK/QPSK phase modulation schemes offer robust performance and simple reception using hard-limiting, albeit at moderate bit-rates. They are frequently used for low-bit rate upstream transmission in FTTC and similar systems, where the receiver (at the remote node) is highly power-constrained. Hard-limiting reception eliminates A/D conversion, reduces internal bitwidths, and can be implemented in a frequency-programmable manner, with low power and cost. A large class of hard-limiting receivers operates on samples of the hard-limited signal. Such sampled hard-limiting receivers are limited in phase-resolution, depending on the harmonic relationship between the carrier frequency and the sampling frequency. Limited phase-resolution can cause considerable loss in margin.
FIG. 1 shows the phase resolution of a BPSK/QPSK receiver as the carrier frequency of the BPSK/QPSK signal changes relative to the sampling frequency (i.e., the rate at which the BPSK/QPSK signal is sampled). The normalized frequency plotted in FIG. 1 is defined as the intermediate frequency F.sub.if divided by the sampling frequency F.sub.s, where the intermediate frequency F.sub.if is defined as follows: If the sampling frequency F.sub.s is less than twice the carrier frequency F.sub.c, then F.sub.if =F.sub.s -F.sub.c ; otherwise, F.sub.if =F.sub.c.
Two numbers are said to be relatively prime when their greatest common divisor is 1. When the sampling frequency F.sub.s is related to the carrier frequency F.sub.c by a ratio of relatively prime numbers and the number representing the sampling frequency F.sub.s is less than about 30, the sampling frequency F.sub.s and the carrier frequency F.sub.c are said to be related by a ratio of small numbers. In that case, interference patterns can develop between the sampling frequency and the carrier frequency, and the ability of a BPSK/QPSK receiver to accurately resolve the phase of the signal can be severely inhibited. These communication problems can occur when the carrier frequency is unknown or when the receiver must work with signals in a wide range of carrier frequencies or both. Such problems can also occur in applications other than FTTC telecommunication systems based on BPSK/QPSK modulation, for example, mobile radio, modem, and other applications in which hard limiting is used prior to time/data extraction.